In the HARQ way, the codeword sent by a sending end not only can detect an error but also can correct the error sometimes. After receiving the codeword, a receiving end detects the error at first, and then corrects the error automatically if the error is within the error correction capability of the codeword; if there are many errors and the errors are beyond the error correction capability but can be detected, the receiving end sends a decision signal to the sending end through a feedback channel to demand the sending end to resend the message. In an Orthogonal Frequency Division Multiplex (OFDM) system, the transmission correctness/wrongness is represented by an Acknowledgement/Negative-Acknowledgement (ACK/NACK) message, so as to judge whether retransmission is required. The transmitted ACK message or NACK message is collectively referred to as HARQ-ACK message.
In a Long Term Evolution (LTE) system (its corresponding standard protocol is Rel-8/9), an HARQ-ACK message can be sent separately in a Physical Uplink Control Channel (PUCCH) or sent with data in a Physical Uplink Shared Channel (PUSCH).
In a Frequency Division Duplex (FDD) system, since uplink and downlink subframes are in a one-to-one correspondence relationship, if only one codeword stream is included in a Physical Downlink Shared Channel (PDSCH), the UE will feed back an HARQ-ACK message of one bit; if two codeword streams are included in the PDSCH, the UE will feed back an HARQ-ACK message of two bits; and if there is no PUSCH for the UE to send in the current subframe, the UE will send the HARQ-ACK message of one bit or two bits in PUCCH in format 1a/1b; while if there is PUSCH for the UE to send in the current subframe, the UE will perform ACK/NACK-state-to-bit mapping, channel coding, scrambling and modulation on the one bit or two bits and then multiplex it with data, so as for transmission in PUSCH.
In a TDD system, since the uplink and downlink subframes are not in a one-to-one correspondence relationship, that is to say, the HARQ-ACK messages corresponding to a plurality of downlink subframes have to be sent in the PUCCH/PUSCH of a single uplink frame, where a set of downlink subframes corresponding to a single uplink frame form a bundling window. In the LTE TDD, two methods for sending the HARQ-ACK message are defined. One is an HARQ-ACK bundling method. The basic idea of this method is to perform logic AND operation on the HARQ-ACK messages fed back in the uplink subframe corresponding to the respective codeword streams corresponding to the downlink subframes. If the PDSCH transmission of one downlink subframe includes two codeword streams, the UE has to feed back a two-bit (each bit corresponding to a respective codestream) HARQ-ACK message subjected to the subframe logic AND operation; if the PDSCH transmission of each subframe only includes one codeword stream, the UE has to feed back an one-bit (which corresponds to the codeword stream) HARQ-ACK message subjected to the subframe logic AND operation; if there is no PUSCH for the UE to send in the current subframe, the UE will send an one or two-bit HARQ-ACK message in PUCCH format 1a/1 b; while if there is a PUSCH for the UE to send in the current subframe, the UE will perform ACK/NACK-state-to-bit mapping, channel coding, scrambling and modulation on the one bit or two bits and then multiplex it with data, so as for transmission in PUSCH. The other one is an ACK/NACK multiplexing method. By this method, it is necessary to feed back one HARQ-ACK message for each downlink subframe, and it is necessary to feed back a plurality of HARQ-ACK messages for a plurality of subframes. When the PDSCH transmission of a downlink subframe includes two codeword streams, the HARQ-ACK message of each codeword is subjected to logic AND operation at first, such that finally each downlink subframe corresponds to one HARQ-ACK message. If there is a downlink subframe which is not scheduled, its HARQ-ACK message is set to a Discontinuous Transmission (DTX) state. That is to say, in an HARQ-ACK multiplexing mode, the HARQ-ACK message can indicate three feedback states: ACK, NACK and DTX. When there is no PUSCH for the UE to send in the current subframe, the UE will send the plurality of HARQ-ACK messages in PUCCH in format 1 b with channel selection. The core idea of this method is to represent the HARQ-ACK messages on different subframes by using different PUCCHs and different modulation symbols on a channel. By this method, an HARQ-ACK message of up to four bits can be carried in PUCCH format 1b with channel selection. LTE defines the mapping table (mapping table for short) between the state combination of the HARQ-ACK messages and the selected PUCCH as well as the two bits b(0)b(1) carried in the channel when different subframes are assigned, namely, when the uplink and the downlink subframes are assigned to be 1:2/3/4 respectively. Here, the three mapping tables are called mapping tables of the LTE.
Whether the UE uses the HARQ-ACK bundling or the HARQ-ACK multiplexing to feed back the HARQ-ACK message is configured by a high layer signaling.
To meet the requirements of the International Telecommunication Union-Advanced (ITU-Advanced), as the evolution standard of the LTE (its corresponding standard protocol is Rel-10), a Long Term Evolution Advanced (LTE-A) system must support an increased system bandwidth (up to 100 MHz), and must be backward compatible with the current LTE standard. Based on the current LTE system, the bandwidth of the LTE system can be combined to obtain an increased bandwidth, which is called Carrier Aggregation (CA) technology. And this technology can improve the spectrum efficiency and alleviate the shortage of spectrum resources of the IMT-Advance system, so as to optimize the utilization of spectrum resources.
In a system in which the CA is introduced, the carrier to be aggregated is called a Component Carrier (CC) or a Serving Cell (SC). Meanwhile, the concepts of Primary Component Carrier/Cell (PCC/PCell) and Secondary Component Carrier/Cell (SCC/SCell) are also put forward. The system in which the CA is performed includes at least one PCell and one SCell, where the PCell is active all the time. In the following description, the component carrier and the Serving cell are equivalent.
When the LTE-A uses the CA technology and a base station configured a plurality of downlink Serving cells for the UE, the UE has to feed back HARQ-ACK messages of corresponding codeword streams of the downlink Serving cells. In the LTE-A, two feedback modes are defined for the transmission of the HARQ-ACK message in PUCCH: a feedback mode of PUCCH format 1b with channel selection and a DFT-s-OFDM based feedback mode.
For an FDD system, format 1 b with channel selection is only applicable to the scenario in which two Serving cells are aggregated, and thus, in this scenario, the codeword stream in each SC corresponds to a respective HARQ-ACK message. The number of bits of the HARQ-ACK message fed back in the FDD system is determined according to the number of assigned Serving cells and the transmission mode, that is to say, when two Serving cells are assigned for the UE, if both the Serving cells are configured in a transmission mode of single codeword stream and the UE uses PUCCH format 1b with channel selection as the HARQ-ACK feedback mode, the UE will use the mapping table A=2 (as shown in FIG. 1), where A represents the number of HARQ-ACK messages corresponding to the codeword streams; if one of the Serving cells is assigned in a transmission mode of single codeword stream and the other is assigned in a transmission mode of dual codeword streams, the UE will use the mapping table A=3 (as shown in FIG. 2), and if both the Serving cells are assigned in the transmission mode of dual codeword streams, the UE will use the mapping table A=4 (as shown in FIG. 3). Here, the mapping tables are called LTE-A mapping table.
For a TDD system, because uplink and downlink subframes are not symmetric, for some uplink subframes, the HARQ-ACK messages of a plurality of downlink subframes have to be feed back in each of them according to different uplink and downlink configurations. In a TDD system in which the CA technology is introduced, a single uplink subframe may be used to feed back the HARQ-ACK messages of a plurality of downlink subframes and downlink Serving cells. In an LTE-A TDD system, PUCCH format 1 b with channel selection can also be used to feed back the HARQ-ACK message. Since PUCCH format 1b with channel selection can support four bits at most, the HARQ-ACK messages fed back have to be bundled in the LTE-A TDD system in a certain form in order that the number of the feedback bits is limited to four and the number of supportable downlink Serving cells is limited to two. It is assumed that time domain bundling is performed but inter-cell bundling is not supported now, but there is still no specific scheme about how to feed back the HARQ-ACK messages of each downlink serving cell and downlink subframe in PUCCH format 1 b with channel selection. Moreover, regarding how to obtain the corresponding PUCCH resources for PUCCH format 1 b with channel selection, there is still no conclusion in the TDD system. Another problem is that there is still no conclusion about whether the LTE-A mapping table used by the FDD is applicable to the TDD system now.